1) Field of the Invention
The present invention relates to a production process of foamed polymer particles.
2) Description of the Related Art
As a foaming process of polymer particles, it has been known to disperse polymer particles containing a volatile foaming agent in a dispersion medium such as water in a closed vessel, to heat the resultant dispersion to a temperature of at least the softening temperature of the polymer particles while maintaining the internal pressure of the vessel at least the vapor pressure of the foaming agent, and then to open the vessel at one end thereof so as to release the polymer particles and the dispersion medium into an atmosphere of a pressure lower than the internal pressure of the vessel. As volatile foaming agents useful in the practice of this process, there have heretofore been used mainly hydrocarbons such as propane, butane and pentane, halogenated hydrocarbons such as trichlorofluoromethane and dichlorodifluoromethane, etc. However, most of these compounds used as the foaming agents have been accompanied by problems of possible hazards such as toxicity and combustibility, of ozonosphere destruction like flons, of impracticability due to their expensiveness, and/or the like. Besides, the volatile foaming agents swell the polymer particles, so that the appropriate range of foaming temperatures upon their foaming is limited. Therefore, they also involve a problem that the foaming temperature greatly affects the expansion ratio of the polymer particles, so that difficulties are encountered on controlling of the expansion ratio.
Many studies have been made to solve such problems. As also disclosed in a method proposed previously by the present applicant, it has also been performed in recent years to use an inorganic gas such as carbon dioxide as a foaming agent (for example, Japanese Patent Publication No. 61227/1987, Japanese Patent Application Laid-Open Nos. 2741/1986 and 4738/1986, etc.).
As described in the above-mentioned publications, it is possible to obtain foamed polymer particles by using an inorganic gas such as carbon dioxide as a foaming agent. However, the use of carbon dioxide as a foaming gent involves a problem that the improvement in expansion ratio is not recognized though an equilibrium pressure within a closed vessel is higher compared with the case making use of a general volatile foaming agent, and the equilibrium pressure within the vessel becomes still higher when making an attempt to increase the expansion ratio. It is hence required to use a vessel capable of withstanding higher pressures as the closed vessel, leading to increase of the cost of equipment. The mere replacement of the volatile foaming agent with carbon dioxide offers little problem so long as polymer particles in a small amount on the order of an experimental scale are caused to expand. However, the expansion ratio lowers as the time goes on from the beginning of foaming if the polymer particles in a great amount on the order of an industrial scale are attempted to expand. For example, the expansion ratio is about 20-30 times at the beginning of foaming, but is lowered to about 10 times near the end of foaming. The scatter of expansion ratio hence becomes very wide. Therefore, such production of foamed particles on the industrial scale has involved some problems to have to solve.